An OBJ file is a standard 3D model format that is used in computer graphics. It stores information about the geometry of a 3D model, including its vertices, faces, and texture coordinates. The structure of an OBJ file is relatively simple, consisting of a series of textual statements that define the various elements of the 3D model. The file begins with information about the vertices, which are the points in 3D space that define the shape of the model. Each vertex is specified using three numbers that represent its X, Y, and Z coordinates. Following the vertex data, the file contains information about texture coordinates, which define how textures are mapped onto the model's surface. Next, the file defines the faces of the model, which are polygons formed by connecting the vertices. Each face statement specifies the vertex and texture coordinate indices that form the vertices of the polygon. Additionally, OBJ files can also store material information, such as the color and texture of the model. This information is defined using the 'mtllib' and 'usemtl' statements, which reference external material library files. Overall, the OBJ file structure is straightforward and versatile, making it widely used in 3D modeling and rendering applications.